Optical coherence tomography (OCT) is a noninvasive, noncontact imaging modality that uses coherence gating to obtain high-resolution cross-sectional images of tissue microstructure. Several implementations of OCT have been developed. In frequency domain OCT (FD-OCT), the interferometric signal between light from a reference and the back-scattered light from a sample point is recorded in the frequency domain typically either by using a dispersive spectrometer in the detection arm in the case of spectral-domain OCT (SD-OCT) or rapidly tuning a swept laser source in the case of swept-source OCT (SS-OCT). After a wavelength calibration, a one-dimensional Fourier transform is taken to obtain an A-line spatial distribution of the object's scattering profile.
OCT provides high contrast 3D visualization of moderately scattering materials using interference of light scattered from a sample (e.g., human eye), with light from a reference surface. Retinal imaging is the most highly developed application for OCT. Thickness information about the retina is highly valuable in the diagnosis of diabetic macular edema (DME), early age-related macular degeneration (AMD), and central serous retinopathy (CSR) among other conditions.
OCT is however expensive. OCT devices are typically complicated instruments including a sample arm with x/y scanner, and an adjustable length reference arm. Very high source brightness is required because is its highly advantageous to have a single spatial mode illuminator, and illumination time is limited by phase washout due to relative motion of the in vivo sample and the reference arm of the system. High spectral resolution is required to achieve a large imaging depth range.
From the inception of FD-OCT, it has been observed that a signal can be decomposed into a portion resulting from the light scattering from the tissue interfering with a dominant reference light and a portion resulting from interfering the light scattering from the tissue interfering with itself. The portion of the signal from sample light interfering with itself has almost uniformly been considered an undesirable image artifact, usually called ‘autocorrelation noise. Various techniques have been adapted over the years to suppress the autocorrelation artifact. The first published account of frequency domain images of the human retina includes a description of a method to modulate the length of the reference arm to attenuate the autocorrelation portion of the signal (Wojtkowski, M., et al. 2002 “In vivo human retinal imaging by Fourier domain optical coherence tomography.” Journal of Biomedical Optics 7 (3): 457-463). The method most commonly used in commercial spectral domain OCT (SD-OCT) systems imaging the eye today use a strong reference power such that any autocorrelation noise is weak relative to the shot noise of the system. Alternatively, in swept source systems, the use of dual balanced detection blocks signal which is common mode, including the autocorrelation artifact.
‘Common path’ OCT systems simplified the OCT beampath to a Fizeau interferometer, such that an optical window in the beam path or even the tissue surface itself could act as the reference surface for interferometric imaging. The important factor for clean imaging was that the reference surface should be dominantly bright such that the autocorrelation of the reflectance profile could be approximated as a correlation of the reflectance profile with a delta function, and thus maintain a very good representation of the reflectance profile of the tissue. In the case where the tissue surface could act as the reference surface, the Fresnel reflection from the air-water interface could be strong enough if the angle of reflection could be captured by the system. In order to supplement this intrinsic reflection, a method was devised to enhance the surface reflection by applying a highly reflective powder to the proximal surface of the organ to be imaged. Such self-referenced systems have cost advantages in terms of system simplicity and have a stability advantage because the systems are relatively insensitive to motion in the axial direction of the sample relative to the imaging system. An obvious drawback, but potential advantage of such tissue surface referenced images is that information about the geometry of the surface is discarded prior to measurement. The image of the tissue is flattened to the reference surface automatically. If the surface geometry does not contain useful information, this flattening makes the signal easier to measure because it contains lower frequency spectral modulations, may make visualization and analysis of the signal easier if the primary interesting information is the tissue thickness from the surface, and finally makes the total image size smaller.
FIG. 1(a) illustrates an example prior-art Fourier domain OCT system without a reference arm (Krstajić, N., Brown, C. T. A., Dholakia, K., and Giardini, M. E. (2012) “Tissue surface as the reference arm in Fourier domain optical coherence tomography.” J. Biomed. Opt 17, 071305-071305). As depicted, a super luminescent diode (SLED) illuminates the tissue via a 3-dB fiber coupler and microscope objective with a working distance of 25 mm and lateral resolution of 17 μm. The reference arm shares the physical path of the sample arm, and is generated by the diffuse surface on the top of the tissue (see FIG. 1(b)). Due to its common path configuration, the single mode fiber (SMF) used can be of arbitrary length. Backscattered spectra from the common arm are detected by a custom transmissive spectrograph containing dispersive optics (diffraction grating (DG) and lenses L1 and L2) and a line CCD detector. Either specular reflection from the air tissue interface or an extrinsic scattering compound is applied to increase reflection and create a dominant peak for correlation existing entirely on the proximal side of the tissue of interest.
It would be acceptable for many applications of retinal OCT to maintain information only about the thickness of tissue layers, and discard information about the tissue surface, such as the curvature of the retina. In fact, most OCT systems do not record the beam pivot geometry with sufficient accuracy to provide good information about the shape of the sphere of the eye, and many analyses of retinal OCT data first flatten the data to simplify or constrain the segmentation process. Maintaining the reference arm to a closely matched pathlength and identical polarization state to the sample arm adds challenges that would be preferably avoided in a low cost OCT system. On the other hand, very few locations on the human retina produce a specular reflection which is directed back towards the pupil and can be used a reference surface for a potential reference arm. Likewise external application of a highly scattering agent across the retinal surface would be unacceptable.